Large-cell Diatom Algae (Image 1)

Microscopic photograph of the large-cell diatom algae Chaetoceros sp., which are part of the annual phytoplankton spring bloom--microscopic algae suspended in water--in the Baltic sea.

Diatoms, a form of phytoplankton, are an important part of the food chain in the sea. The larva of many species of saltwater fish feed on copepods (small planktonic crustaceans) and the copepods, in turn, feed on diatoms. Changes in climate could affect the seasonal cycles of these creatures, which could in turn affect the balance between predator and prey.

Ulrich Sommer, professor of biological oceanography at the Leibniz Institute of Marine Sciences at the University of Kiel, is studying whether increases in water temperature will significantly alter the time of the spring bloom. Experiments were conducted with indoor mesocosms filled with late-winter plankton from the Baltic Sea. Sommer found that increases in water temperature altered the time of the spring bloom only slightly--about one day-per-degree centigrade. However, an increase in water temperature accelerated the hatching time of the nauplius larvae from over-wintering copepod eggs by about nine days-per-degree centigrade.

In the lab, the Chaetoceros sp. were dominant under cold conditions during experimentally induced spring blooms. But when the water temperature increased, the types of species that make up the spring bloom changed from large-cell diatom algae to small flagellates. This could potentially affect the food chain because the latter are rarely eaten by copepods.

If an increase in water temperature causes the period of maximum food supply and food demand for nauplius larvae to spread further apart, and the composition of species in the spring bloom becomes increasingly inadequate, this will alter the balance of the food chain, and could mean the nauplii would die of starvation and the fish larvae in turn would have nothing to eat.